I own a Nikon D80 with Tamron 18-200mm lens. I'm looking to experiment with using colored gels like these Rosco ones but am not sure what type of holder I could use to hold the gel in place (I've tried rubber bands and tape, neither quite work), or if one even exists. Every time I search gel holders, I get lighting gel holders. Can anyone tell me what and where I can search?
Look at gel filter holders made by Lee. http://www.leefilters.com/index.php/camera-directory/camera-dir-list/category/gelsnap-filter-holder
You could go the DIY route very inexpensively. In solar photography, a common solar filter is a mylar film, similar in thickness and flexibility to filter gels. Many people create their own solar filter holders. Examples to give you some ideas:
The author cut up a cardboard folder into two 4" × 4" squares and a cylindrical tube that slides over the lens. Both of the 4" squares had a hole cut in the center of them matching the diameter of the cylinder, and one of the squares was glued to the tube. He sandwiched the thin film filter between the two squares, using clear (Scotch) tape to hold the squares together.
This author's design has the benefit of stacking very flat when not in use. He used several pieces of 5mm thick foam core board (polystyrene foam board sandwiched between sheets of stout paper). He created a mounting flange by stacking and gluing two small squares a couple centimeters larger than the lens's outer diameter, and an additional larger sheet of about 4" square or more. A hole the size of the lens diameter is cut into the stack. He uses a rubber band on the end of the lens to provide a friction fit for the flange to slide over. An additional 2 large squares, with a large hole cut in each of them, are used to sandwich the filter sheet, held together with double-sided tape. The mounted filter is held to the large square on the flange with binder clips.
Note: This is the method he uses for telescope filters. To mount a thin film filter to a camera lens, he still mounts the filter between 2 sheets of foamcore board. But he attaches the filter to a Cokin square holder, which is mounted as normal to the lens. However, the telescope method will work for camera lenses as well.
This method, from AstroSolar.com, is basically the same as the first method: using stiff cardboard, make a cylindrical sleeve to slide over the lens. Attach a large flange sheet of cardboard to the cylinder, and use another sheet of cardboard to sandwich the filter film between it and the flange.
Personally, I would probably use a step-up ring to match my largest lens filter thread, stepping up a large amount. I would glue thick sturdy card stock or foam core board to the step-up ring to make a mounting flange, and use the technique of sandwiching the filter film between 2 squares of card stock or foamcore board, and use binder clips to attach the mounted filter to the flange board.